MHX2 import in 2.79 and 2.80?
I can now answer my own question.  The mhx2 importer DOES install and enable under Blender 2.79.  What threw me off is that A: the error message I've quoted in my original question still appears when it's enabled, and B: the mhx2  file import no longer appears in the Import/Export dialogue where I expected to find it.  Instead, it is in an (apparently) new "mhx2 runtime" tool tab.  "Import MHX2" is now found in this tool tab.